Abstract A generalization of the phase-field theory for multivariant martensitic phase transformations is suggested that allows one to vary martensite–martensite interface energy independent of energy for austenite–martensite interfaces. The finite element method is utilized to solve the coupled phase-field and elasticity equations. Width and energy of the austenite–martensite interfaces are determined. Splitting of the martensite–martensite interface into two austenite–martensite interfaces, leading to barrierless austenite nucleation, is obtained. The effect of the martensite–martensite interface energy and grain size on the stationary and non-stationary nanostructure inside the transforming grain embedded in the austenitic matrix is determined. Some nano-structures differ essentially from the prediction of crystallographic theory. Relationships between the number of twins in grain vs. grain size, and width of twin vs. its length are found. Two unexpected stress-relaxation mechanisms at the boundary of ...